Joel Madden and Nicole Richie

Longtime couple Nicole Richie and Joel Madden are engaged, People reports.

Richie, 28, showed off her ring and announced the engagement during a Monday taping of The Late Show with David Letterman, a source told the magazine. The episode is scheduled to air Friday, but Madden, 30, couldn't keep the news to himself for long.

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"Yep. I'm engaged. Very happy. Yeah we've been engaged for a while so [you're] all [kinds] of late on that. But Thanks for the hooplah all the same," he tweeted Monday, thanking his family and friends for not letting the news slip.

He later deleted the tweets — which can still be seen here — and wrote Tuesday morning, "Just woke up. I'm screwed. Oh boy."

Calls and e-mails to Richie's rep were not immediately returned.

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Together since 2006, Richie and Madden have two children together, 2-year-old daughter Harlow and 5-month-old son Sparrow.